title: Influence of transplant size on the above- and below-ground performance of four contrasting field-grown lettuce cultivars.
abstract: OBJECTIVE
  Modern lettuce cultivars underperform under conditions of variable temporal and spatial resource availability, common in organic or low-input production systems. Information is scarce on the impact of below-ground traits on such resource acquisition and performance of field-grown lettuce; exploring genetic variation in such traits might contribute to strategies to select for robust cultivars, i.e., cultivars that perform well in the field, even under stress.
  
  
  METHODS
  To investigate the impact of below-ground (root development and resource capture) on above-ground (shoot weight, leaf area) traits, different combinations of shoot and root growth were created using transplants of different sizes in three field experiments. Genetic variation in morphological and physiological below- and above-ground responses to different types of transplant shocks was assessed using four cultivars.
  
  
  RESULTS
  Transplanting over-developed seedlings did not affect final yield of any of the four cultivars. Small transplant size persistently impacted growth and delayed maturity. The cultivars with overall larger root weights and rooting depth, "Matilda" and "Pronto," displayed a slightly higher growth rate in the linear phase leading to better yields than "Mariska" which had a smaller root system and a slower linear growth despite a higher maximal exponential growth rate. "Nadine," which had the highest physiological nitrogen-use efficiency (g dry matter produced per g N accumulated in the head) among the four cultivars used in these trials, gave most stable yields over seasons and trial locations.
  
  
  CONCLUSIONS
  Robustness was conferred by a large root system exploring deep soil layers. Additional root proliferation generally correlates with improved nitrate capture in a soil layer and cultivars with a larger root system may therefore perform better in harsh environmental conditions; increased nitrogen use efficiency can also confer robustness at low cost for the plant, and secure stable yields under a wide range of growing conditions.
authors.affiliation: Laboratory of Plant Breeding, Department of Plant Sciences, Wageningen UR Wageningen, Netherlands ; Department of Plant Sciences, Centre for Crop Systems Analysis, Wageningen UR Wageningen, Netherlands.
authors.forename: P J
authors.initials: PJ
authors.lastname: Kerbiriou
authors.forename: T J
authors.initials: TJ
authors.lastname: Stomph
authors.forename: E T
authors.initials: ET
authors.lastname: Lammerts van Bueren
authors.forename: P C
authors.initials: PC
authors.lastname: Struik
keywords: lettuce; nutrient use efficiency; root activity; transplanting